1. Field of the Invention
The present invention relates to a machine tool capable of grinding a workpiece such as a machining center having a grinding capability, a grinding center and the like, and more particularly to a machine tool capable of grinding a workpiece with a truing apparatus for truing a grinding wheel of the machine tool.
2. Discussion of the Prior Art
A machine tool of the above-mentioned type is generally used for grinding a workpiece. Such machine tool is provided with a table movable in a first horizontal direction, a saddle supported by a column for movement in a second horizontal perpendicular to the first horizontal direction, and a spindle head supported by the saddle for movement in a vertical direction. A cylindrical grinding wheel having a cylindrical side surface and a bottom end surface is attached to a spindle supported by the spindle head for rotation with the spindle. The grinding wheel generally contains hard abrasive grains such as CBN (Cubic Boron Nitride) abrasive grains or diamond grains. A workpiece is mounted on the table to be ground by the grinding wheel.
In such machine tool, the cylindrical side surface and the bottom end surface of the grinding wheel are caused to contact with the workpiece to grind the workpiece. During such grinding operation, the cylindrical side surface and the bottom end surface of the grinding wheel gradually wear away and get rough. Therefore, the cylindrical side surface and bottom end surface of the grinding wheel must be trued to keep predetermined shapes.
The machine tool is therefore provided with a truing apparatus 2 on a table 1, as shown in FIGS. 1 and 2, and the grinding wheel 3 is trued with the truing apparatus 2. Namely, the base 4 of the truing apparatus 2 is mounted on the table 1, and the body 5 thereof is mounted on the base 4 and extends upwardly along an inclined direction intersecting with a horizontal plane with an angle of 45.degree., as shown in FIG. 2. A mounting shaft 6 is supported by the body 5 for rotation about an axis parallel to the inclined direction. A base plate 7a of a truer 7 is detachably fixed to the upper end portion of the mounting shaft 6, which protrudes from the body 5. An abrasive layer 7b is bonded at the periphery of the base plate 7a, as shown in FIG. 3, and a first truing surface 7c and a second truing surface 7d, which perpendicularly intersect with each other, are formed at the periphery of the abrasive layer 7b. The first truing surface 7c becomes parallel to the horizontal plane at the truing position TP, while the second truing surface 7b becomes parallel to a vertical plane at the truing position TP.
During a truing operation, the truer 7 is rotated by a motor through the mount shaft 6 while the grinding wheel 3 is also rotated. Under such condition, the grinding wheel 3 attached to the spindle is moved to contact with the first and second truing surfaces 7c and 7d of the truer 7. Namely, the bottom end surface of the grinding wheel 3 is trued using the first truing surface 7c of the truer 7 while the cylindrical side surface of the grinding wheel 3 is trued using the second truing surface 7d. The relative movement of the truer 7 with respect to the grinding wheel 3 is accomplished by movements of the saddle and the spindle head.
Further, the machine tool is provided on its table 1 with a touch sensor 8 for automatically measuring the diameter of the grinding wheel 3. The truing apparatus 2 and the touch sensor 8 are arranged on one end of the table 1 with a predetermined space in a direction perpendicular to the movement direction of the table 1, as shown in FIG. 1.
After truing operation, the probe 8a of the touch sensor 8 is brought into contact with the trued side surface of the grinding wheel 3 at one side thereof and then at the other side thereof with respect to the rotational axis of the grinding wheel 3. The diameter of the grinding wheel 3 is calculated based upon two positions at which the probe 8a comes into contact with the side surface of the grinding wheel 3. The diameter thus calculated is used for controlling the movement of the saddle in a next truing operation.
Such machine tool, however, has a problem that whole area of the upper side of the table 1 cannot be used effectively, because part of the table 1 is occupied by the truing apparatus 2 and the touch sensor 8. Namely, a space for supporting a workpiece is limited so as to prevent the workpiece and fixtures clumping the workpiece from interfering with the truing apparatus 2 and the touch sensor 8.
Also, the machine tool has a problem that the truing apparatus 2 cannot true the grinding wheel 3 accurately, because the truer 7 bends during truing operations. Namely, the abrasive layer 7b of the truer 7 has a thin thickness in a direction parallel to its rotational axis, and the rotational axis inclines with respect to the rotational axis of the grinding wheel 3 with an angle of 45.degree.. Therefore, the abrasive layer 7b comes into contact with the bottom end surface of the grinding wheel 3 with an inclined posture when truing the bottom end surface, as shown in FIG. 3. As a result, the abrasive layer 7b tends to be deformed due to a truing resistance between the abrasive layer 7b and the grinding wheel 3, as indicated by a broken line in FIG. 3. This problem occurs in both of the truing operation for the side surface of the grinding wheel 3 and the truing operation for the bottom end surface of the grinding wheel 3. More specifically, the abrasive layer 7b tends to bend outwardly when the truer 7 is moved in a direction in which one edge having an obtuse angle precedes, while the abrasive layer 7b tends to bend inwardly when the truer 7 is moved in a direction in which the other edge having an acute angle precedes. Since these deformation of the abrasive layer 7b causes vibrations or other instable conditions, it is difficult in the conventional machine tool to carry out accurate truing operations.